Dimethyl hydrogen phosphite (DMHP) has been found to be a lung carcinogen in F344/N rats but not in B6C3F1 mice. This study was designed to investigate the in vitro metabolism of DMHP by liver and lung preparations from rats and mice. The purpose of the study is to examine the role of metabolism in tissue and species selectivity to the toxic effects of DMHP. Preliminary results showed that DMPH is metabolized to formaldehyde by rat liver and lung microsomal mixed function oxidase systems. This metabolism was found to be NADPH dependent and the level of formaldehyde formation increased as the incubation time increased up to 2 hrs.